In 1015 fetuses undergoing first-trimester karyotyping because of increased nuchal translucency thickness, the incidence of chromosomal abnormalities increased with both maternal age and nuchal translucency thickness. The observed numbers of trisomies 21, 18 and 13 in fetuses with nuchal translucency thicknesses of 3 mm, 4 mm, 5 mm and > or = 6 mm were approximately 3 times, 18 times, 28 times and 36 times higher than the respective numbers expected on the basis of maternal age. The incidences of Turner syndrome and triploidy were 9-fold and 8-fold higher but the incidence of other sex chromosome aneuploidies was similar to that of an unselected population of women undergoing first-trimester fetal karyotyping for maternal age. In the chromosomally normal group, the incidence of structural defects, mainly cardiac, diaphragmatic, renal and abdominal wall, was approximately 4%, which is higher than would be expected in an unselected population. The rates of fetal loss in the groups with nuchal translucency thickness of 3 mm and 4 mm were 2% and 4%, respectively, which is similar to the 2.3% rate of fetal loss observed in a group of fetuses with normal nuchal translucency thickness undergoing chorion villus sampling. For fetal nuchal translucency thickness of > or = 5 mm, the rate of fetal loss was 13%.